The present invention relates to a method for producing improved water-absorbent resins.
Water-absorbent resins are used in the manufacture of sanitary napkins, tampons, diapers, disposable dustcloths and other sanitary goods, and as water-retaining agents in the fields of agriculture and horticulture. They are also used for the purpose of coagulation of sludge, prevention of dew condensation on building materials, dehydration of oil, and so on.
Known water-absorbent resins of this type include crosslinked carboxymethylcellulose, partially crosslinked polyethylene oxide, hydrolyzates of starch-acrylonitrile graft copolymers, partially crosslinked polyacrylic acid salts, vinyl alcohol-acrylic acid salt copolymers, and so on. However, each of them has its drawbacks, such as unsatisfactory absorbency, poor gel strength in spite of high absorbency (if attained), formation of sticky gel by water absorption, or slow rate of water absorption.
It is known that the rate of water absorption can be increased by increasing the density of crosslinking of a water-absorbent resin, thereby lowering the water-absorbency thereof. However, this method is undesirable, since the absorbency which is the most important characteristic of the water-absorbent resin is reduced, because the crosslinking density becomes too high.
Another known method of increasing the rate of water absorption of a water-absorbent resin comprises admixing the water-absorbent resin with water in the presence of a hydrophilic organic solvent such as a monohydric lower alcohol to dissolve or disperse water in the alcohol, whereby water is absorbed by the resin substantially uniformly, crosslinking the resin with water substantially uniformly absorbed therein, and then drying. In practicing this method, it is considered preferable from the viewpoint of the characteristics of water-absorbent resin to conduct the crosslinking in a state in which a large amount of water is absorbed by the resin. However, in practice, the amount of water is limited and, in addition, aggregation of resin particles which are in the swollen state may occur even when the amount of water absorbed by the resin is small, thus easily leading to lump formation. The method is thus poor in workability, which renders the method less suited for commercial purposes. Therefore, it is necessary, in practicing the method, to place water-absorbent resin particles in a water-swollen state by adding a small amount of water in the presence of a large amount of a hydrophilic organic solvent, thereby preventing aggregation of resin particles, which otherwise will occur, during crosslinking. If conducted in such a manner, this method will encounter such problems as high production cost and low productivity.
It is an object of the present invention to provide a modified water-absorbent resin having a good water absorbency and a high water absorption rate and capable of forming a gel having a high strength and nonstickiness by water absorption.
A further object of the present invention is to provide a method for producing the modified water-absorbent resin easily and inexpensively in a good efficiency.
These and other objects of the present invention will become apparent from the description hereinafter.